1) Field of the Invention
The present invention relates to touch screen technology and, more particularly, to a system and method that provides touch sensitivity to existing display panel technology while requiring fewer manufacturing steps, utilizing much of the existing display structure and resulting in reduced cost and a thinner overall panel assembly compared to existing touch sensitive display panels.
2) Description of the Art
Modem LCD display technology is well known in the art. Briefly, Liquid Crystal Display (LCD) technology uses two clear panels which orthogonally polarize light. Sandwiched between the panels is a layer of liquid crystal material which can change the polarized direction of light. In quiescent state, and assuming twist nematic (TN) liquid crystal, the panel appears clear since the relaxed liquid crystal twists the polarization vector of the light from one polarized panel through 90 degrees to match the other polarized panel. When an electric field is applied to the liquid crystal layer, the polarization twist imparted by the liquid crystal layer is made correspondingly less (as the field strength increases). Thus less light passes through the panel and the intensity of the passing light is governed by the completeness of the twist of the polarizing vector of the light through the liquid crystal material layer (which is inversely proportional to the electric field applied to the liquid crystal). Modern LCD displays use this phenomenon at the pixel level for each of the primary colors, red, green and blue. By closely controlling the electric field applied to the liquid crystal, the amount of ‘twist’ given to the polarized light as it passes through the liquid crystal can be tightly controlled. Thus, the intensity (transmissibility) of the red, green, and blue light for each pixel can be tightly governed. To the viewer, this gives the illusion that a pixel takes on any of myriad colors. For information on typical LCD display systems, see the following, each of which is incorporated herein by reference: “Optics of Liquid Crystal Displays,” Yeh and Gu, Wiley-Interscience (September 1999); “Display Systems: Design and Applications,” MacDonald and Lowe, John Wiley and Sons (June 1997).
There are several available architectures known for creating and manufacturing the pixels for display panel technologies. Of these, they can be generalized into one of two major categories: one is passive array technology and the other is active array or active matrix technology. An example of typical passive array technology LCD is shown in FIG. 8 (this is also the typical passive architecture for electronic paper, such as that sold under the Sony trademarks “Sony Reader” and “LIBRIe”, which does not require the polarizing plates). A first clear panel 802 polarizes light in one direction. A second clear panel 804 has disposed on it a number of clear electrical conductors (typically ITO) 812, arranged in a grid fashion and having traces running to the periphery of the panel. A third panel 806 is typically an encapsulated nematic liquid crystal layer, which is followed by another panel 808 that is covered with a clear conductive material forming (this may be opaque for electronic paper) a common electrical plane. The last panel 810 in the FIG. 8 acts to polarizes light orthogonally to the first panel 802. Pixel conductors 812 along with the common electrical plane 808 form a plurality of pixel plate capacitors with the liquid crystal layer being the dielectric of each.
Light passing through panel 802 is polarized in a given direction (i.e., filtered to be polarized). The polarized light continues on and passes through panels 804 and at 806, where the polarized light is ‘twisted’ by the liquid crystal layer so that when it emerges from the other side, it is polarized orthogonally to that when it passed through the first panel. The light continues through panels 808 and 810, since it is now polarized in the direction of polarization of panel 810. Thus, the panel appears relatively clear. Upon application of a charge (relative to panel 808) to any of the pixels 812, the underlying liquid crystal layer will ‘twist’ the light less than it did in its relaxed state. Thus, these pixels appear darker owing to less light transmission through them. As the pixel charge (i.e., electric field strength) is increased, the liquid crystal between the charged pixel 812 and the common electrical plane twists the light less and less until the light is blocked from passing through the panels. In this way, the transmissibility of the panel is electrically controllable.
FIG. 9 shows another common passive matrix array typically used for OLED, PLED, etc. In this architecture, there are a first plurality of conductors 902 oriented orthogonally to a second plurality of conductors 904. Sandwiched between the first and second sets of conductors is an emissive material layer 906 (shown as an entire layer in the figure, but it may also be disposed only at conductor overlapping points if desired). Typically, at least one of the sets of conductors is transparent. A potential difference is applied to at least one of conductors 902 and 904, causing a current to flow through the emissive material layer at the point of overlap between the conductors having the potential difference. This then causes the emissive material to illuminate. For information on quantum dot displays technology, see the following references, incorporated herein by reference: U.S. Pat. No. 6,992,317, U.S. Application 20050006656, and “Optics of Quantum Dots and Wires,” by Garnett W. Bryant (Editor), Glenn S. Solomon (Editor), Artech House Solid State Technology Library (2004).
Passive arrays are typically used for very small arrays. This is because for the array of FIG. 8, each pixel typically has a connection to the periphery of the panel, and this grows exponentially with the number of pixels. The array of FIG. 9 is essentially a multiplexed array wherein each pixel is illuminated for only that time that it is electrically stimulated. Since each pixel typically shares connections with many other pixels, they must each be stimulated in time-multiplexed fashion and rely on eye persistence to produce a smooth image. This tends to produce a dimmer image than could be obtain by each pixel being illuminated for the duration of the display time.
Therefore, current preference in the art is to use active matrix display technology owing to it being generally brighter and requiring fewer (compared to a passive array having the same number of pixels) external connections for some implementations. Active matrix LCD technology uses thin film transistor technology, or TFT as it is known in the art, for selectively coupling driver circuitry to each pixel capacitor in the LCD array. Using a grey scale LCD panel for illustration only, and referring to FIG. 1, in this technology, a first clear panel (a substrate panel) has a number of transparent conductor electrodes 10 applied to the clear panel in a grid arrangement with each electrode corresponding to a pixel. Each of these electrodes forms one tiny plate of a capacitor corresponding to each pixel (i.e., a pixel plate capacitor), and each of these plates is connected to the source of an associated small thin-film transistor 16, which is also on the clear panel (i.e., the substrate panel). In another construction technique, the electrodes are formed of an opaque material with the center cut out to allow light to transmit through, however, transparent electrodes admit more light as a whole. In yet another construction technique, the pixel is subdivided as described above with each subpixel having its own TFT transistor.
To increase the charge storing capacity of the pixel, capacitor 18 is typically placed in parallel with the pixel plate capacitor formed by plate 10 and a transparent common electrical plane (not shown). Running through the matrix of electrodes and capacitors is an X-Y grid of conductors, 12 and 14, with one direction connected to the drain of each transistor in a row and the orthogonally-running conductors connecting to the gate of each transistor in a column. Thus, each pixel can be manipulated by proper coordination of signals on the X-Y grid of conductors.
For more detailed information on active matrix LCD techniques see the following references, incorporated herein by reference: “Optics of Liquid Crystal Displays,” Yeh and Gu, Wiley-Interscience, September 1999; “Display Systems: Design and Applications,” MacDonald an Lowe, John Wiley and Sons, June 1997; “Active Matrix Liquid Crystal Displays: Fundamentals and Applications,” Boer, Newnes, September 2005; “TFT/LCD Liquid Crystal Displays addressed by Thin-Film Transistors,” Tsukada, CRC Press, June 1996; U.S. Pat. No. 6,372,534; U.S. Pat. No. 6,956,632; U.S. Pat. No. 6,819,311; U.S. Pat. No. 6,115,017; U.S. Pat. No. 5,204,659.
The panel having the pixel plates, TFT transistors and X-Y conductors as described above (i.e., the substrate panel) is coupled to a film or panel which is designed to polarize light in a certain direction (or the substrate layer itself may perform this function. A second clear panel (not shown) is designed to polarize light in an orthogonal direction to the first panel. This second panel is also covered with a clear electrode which typically forms the common electrical plane, and is the other plate for all pixel capacitors (the first plates of which are on the substrate panel). Sandwiched between the first and second panels is a thin liquid crystal layer, which is responsive to the charge on each pixel capacitor thus formed and is the dielectric material for each pixel capacitor. This liquid crystal layer performs the twisting of the polarizing vector of the light which passes through it. The amount of polarization twist imparted to passing light for each pixel location is inversely proportional to the charge in each pixel plate capacitor (i.e., the electric field strength) (which may be augmented by the additional auxiliary capacitor as described above) which is applied to the liquid crystal material forming the dielectric layer of the pixel capacitors as described previously.
To extend the above description to color, the number of pixels is trebled to achieve the same resolution (as a corresponding grey scale panel) and each pixel is associated with red, green or blue light (usually by a filtering structure or layer). In this way, 3 pixels, one each of red green and blue are perceptively combined to form an image pixel of the rendered image in full color.
Other types of liquid crystal displays are known in the art including IPS (in-plane switching) and VA (vertical alignment). These have found favor recently in that they allow wider viewing angles than TN (twist nematic). The basic principles of operation of these types are known and will not be reviewed here.
Owing to the fact that the pixel capacitor charges will dissipate with time and that the image on the display may change from time to time, the pixel capacitors are refreshed from time to time with new values/data. Typical refresh rate for LCD technology is about 60 times a second for the whole of the display.
While typical LCD technology stores an analog value of charge on the pixel capacitor plate corresponding to the desired brightness of the pixel (i.e., an analog value), a pixel design has recently been developed where the total area of the pixel is subdivided into subpixels. For example, see FIG. 4, which does not show the auxiliary storage capacitors. These subpixels are in the ratio of half, quarter, eighth, sixteen, etc. (40a, 40b, 40c, 40d) of the total pixel area, 40. In this way, appropriate combinations of subpixels can simply be turned on and off to achieve the desired brightness of the overall pixel. The advantage is that now the subpixels need only have two states, full on and full off. The modulation of the overall pixel brightness is achieved by proper selection of the subpixels to turn on. This is accomplished via bus 48 and transistors 44. With this technique, each pixel essentially stores a digital value for its brightness and the effects of voltage droop on the pixel plates between refresh cycles are reduced since the subpixel capacitors are slightly overcharged (e.g., driven beyond subpixel saturation). Saturation in this context generally means that the pixel is at minimum transmissibility, and an increase in pixel capacitor voltage will not further decrease the transmissibility of the pixel. For more information on this display architecture see the following references, each of which is herein incorporated by reference: U.S. Pat. No. 6,956,553, U.S. Pat. No. 5,124,695.
Some implementations of display panels using existing LCD technology also incorporate touch screen technology which allows a user to indicate a position on a screen by simply touching the screen with his finger or a stylus and having the point of contact sensed and translated by electronics to an indication of position. Essentially, the technology turns the screen into a mouse pad for such things as tablet PCs, personal digital assistants, cell phones, etc. There are several forms of the technology which are basically add-on systems to standard LCD technology to make the screen touch sensitive.
In resistive touch screen technology, a resistive/conductive film (usual two layers) is applied to the surface of the LCD and the film(s) is flooded by associated electronics with a relatively constant current. When contact is made with the film, disruption in the current flows within the film is detected by associated electronics which quickly pinpoints the area of the disruption. Thus, the information can be used to determine the location on the screen to which the user is touching. For examples of resistive touch screen systems, see the following references, each of which is incorporated herein by reference: U.S. Pat. No. 6,841,642; U.S. Pat. No. 6,781,579; U.S. Pat. No. 6,424,094; U.S. Pat. No. 6,246,394; U.S. Pat. No. 6,624,835; U.S. Pat. No. 6,204,897; U.S. Pat. No. 6,559,835; U.S. Pat. No. 6,163,313.
Similarly, in capacitive touch screen technology, a film is applied to the LCD and is flooded with a uniform charge by associated electronics. When touched, the charge is disrupted. Associated electronics again sense the disruption and quickly pinpoint the location of the disruption and determine the screen location being touched. For examples of capacitive touch screen systems, see the following references, each of which is incorporated herein by reference: U.S. Pat. No. 6,819,316; U.S. Pat. No. 4,922,061; U.S. Pat. No. 4,853,498; U.S. Pat. No. 4,476,463; U.S. Pat. No. 5,194,862. Another type of capacitive touch screen applies conductors over the surface of the screen and has associated electronics operable for detecting the capacitive coupling between conductors when the finger or other object is in proximity or contact with the screen. For examples of this type of touch screen, see the following references, each of which is incorporated herein by reference: U.S. Pat. No. 6,961,104; U.S. Pat. No. 6,825,833.
Other technologies include RF, optical, surface acoustic wave, infrared, etc. Generally, all these technologies work by flooding the surface of the LCD with some kind of field (current, charge, RF, etc.), detecting disruptions in the field, and determining the position of contact based upon analysis of the disruption(s) in the uniform field or by having sensors disposed on the display which sense a field emitting device, such as an RF generating “pen” (like those used on familiar tablet PCs). Each of these existing technologies can be considered to be “add on” technologies, in that they all require additional manufacturing steps in the creation of the LCD display panel or are technologies applied to the LCD panel after the LCD panel is made. For further examples of these types of touch screen systems and others, see the following references, each of which is incorporated herein by reference: U.S. Pat. No. 6,172,667; U.S. Pat. No. 5,708,460; U.S. Pat. No. 6,411,344; U.S. Pat. No. 6,369,865; U.S. Pat. No. 6,961,015; U.S. Pat. No. 6,741,237; U.S. Pat. No. 6,506,983.
Recently, U.S. Pat. No. 6,885,157 (herein incorporated by reference), purports to describe an integrated touch screen and OLED flat-panel display wherein the OLED display has a plurality of electrical conductors disposed on its surface for touch sensing. See also U.S. Pat. No. 6,841,225. U.S. Pat. No. 6,512,512 (both herein incorporated by reference), which shows a touch sensor where actual contact between layers is detected upon touch contact. U.S. Pat. No. 5,777,596 (herein incorporated by reference) shows an LCD display panel wherein the charge time of pixels is used to give an indication of the pixel capacitance and used to determine touch contact with the display panel. However, measuring the charge imparted to such a small capacitance is cumbersome and susceptible to large measurement errors and may require display pixels to be disabled from their primary purpose (that of display) so that the time required to impart a known amount of charge to the pixel can be determined. A better approach, described more fully below, is to use AC analysis techniques rather than time domain techniques.
Also known in the art is U.S. Pat. No. 4,224,615 (incorporated herein by reference), which compares the impedance of a display element to a reference cell impedance to detect changes in capacitance of the display element when under pressure from touch contact. However, this reference does not teach how the impedance measurement is to be made, or that there is any beneficial time at which the impedance measurement or comparison should be performed, or further, that there is a frequency preferred for the impedance measurement, etc. It is important to bear in mind that at the time of the issuance of U.S. Pat. No. 4,224,615, LCD display images were typically relatively static, being a watch display, calculator display, etc. As is well known, in order to achieve sufficient contrast and to not electrolyze the liquid crystal material, LCD displays typically have the signal polarity applied to the liquid crystal layer reversed from time to time. This is effectively a low frequency AC signal and while it may be used to measure impedances, it is not of sufficiently high frequency to achieve a high sampling rate necessary for position sensing of large modern displays with rapidly changing images.
U.S. Pat. No. 4,363,029 (incorporated herein by reference), describes proximity detection for a display that operates by comparing pixel capacitance variations caused by the proximity of a person's finger to the display with a reference capacitance element incorporated in the device. However, the display element drive is taken to an inactive (non-display) state when sensing and comparing the pixel capacitances to a reference capacitance. U.S. Pat. No. 4,841,290 (incorporated herein by reference), uses an external pen to sense magnetic fields caused by the scanning of the underlying display image and comparing the phase of the sensed fields to that of the scan drivers to determine location of pen contact on the display. This is similar to optically-based position sensing pencils, known in the art, and used with CRT displays wherein the position the pen is contacting the display is determined by a photo sensor in the pencil detecting the minute increase in brightness caused by the pixel being refreshed and this information being coordinated with the display's refresh information. In both cases, an external device is used to perform such sensing.
U.S. Pat. No. 5,043,710 (incorporated herein by reference) uses a high frequency generator and integrator (i.e., filter) to detect changes in the dielectric constant (i.e., the sudden delta of the dielectric constant) of the liquid crystal layer of an LCD when subject to a change in the applied physical pressure such as that created when a finger presses the display screen. Basically, when the high-frequency stimulated liquid crystal layer is compressed, a charge is created (released by the dielectric) which is integrated (i.e., filtered) to give a pulse indication of touch contact. This method is effectual for sensing when a localized change in pressure of the liquid crystal layer occurs and only when a change in pressure occurs (i.e., for sensing a delta in pressure). If the measurement is made too long after the change occurs it can be missed. Additionally, if the object remains in pressure contact with the display screen (i.e., continually touching), its continual contact may not be sensed (owing to there being no ‘change’ or delta to detect). Another problem with the method is that the high frequency AC is continually applied to the electrode that is common to all display elements and it shares its signal return with the display control circuitry. Thus, there will always be some ac current flowing through all the display elements (capacitors) and into the controller electronics of the display even when not sensing. This can amount to quite a large current always flowing in displays having a large number of pixels or being of large area (and is directly proportional to the frequency of the AC source). This current wastes energy and creates an electrically noisy operating state for the display electronics. In addition, owing to dielectric losses, the dielectric layer (i.e., the liquid crystal) will heat up which could shorten its life significantly.
U.S. Pat. No. 6,133,906 (incorporated herein by reference), uses a pen that generates a signal that is magnetically, capacitively, or directly coupled into the underlying display (i.e., pixel) electrodes, wherein the signal is preferably AC so as to not disrupt the underlying display image and wherein the signal travels along the paths and circuits of the underlying display for detection by associated electronics. However, again, this method uses an external device to generate special fields which are coupled into the electrical connection structures of the display.
Application of a touch sensitive technology and associated electronics to an LCD display, for the purpose of manufacturing a touch sensitive display panel, has in the past involved added cost and steps and increased the thickness of the display panel with applied films or extra panels or conductors (which usually dim the display as well). Additionally, many approaches to adding touch sensitivity known in the art have required the use of specialized external hardware. While many attempts have previously been made to integrate touch-sensing capability within the display, each has, for one reason or another, been left wanting. The need exists for a more complete integration of the touch-sensing capability within the display technology, and further, to make better use of the regular conductor structure and electrical nature and properties of prior art flat panel display technology. The need further exists for better touch detection and position determination techniques when using the display panel's inherent structure and properties while minimizing perturbation to the displayed image.